1. Field of the Invention
This invention relates to an internal combustion engine of the type including a plurality of cylinders split into first and second groups and adapted to operate in a split cylinder mode with the first group of cylinders held operative and the second group of cylinders suspended when the engine is under low load conditions and in a full cylinder mode with both of the first and second groups of cylinders held operative when the engine is under high load conditions. The invention is more particularly concerned with means for use in such an engine for preventing exhaust gases recirculated in a second branch of the intake passage leading to the second group of cylinders from flowing therefrom into a first branch of the intake passage leading to the first group of cylinders when the engine is shifted from a split cylinder mode to a full cylinder mode.
2. Description of the Prior Art
It is well known that when an engine is operating under a higher load condition, its fuel combustion and fuel economy becomes higher. In view of this fact, split operation control systems have already been proposed for use in multicylinder internal combustion engines such as automotive vehicle engines and the like subject to frequent load variations. Such a system is responsive to engine low load conditions for cutting off the supply of fuel to some of the cylinders of the engine so as to hold them thereby maintaining the load of each of the other operating cylinders above a predetermined level and attaining high fuel economy.
Assuming that such a split operation control system is applied to a 6-cylinder internal combustion engine for cutting off the supply of fuel and fresh air to three cylinders of the engine so as to suspend them when the engine is under low load conditions, air will be discharged from the suspended cylinders and mixed with exhaust gases discharged from the other three operating cylinders, which results in a reduction of temperature of exhaust gases passing through a three-way catalyzer provided in its exhaust system, causing poor exhaust emission purifying performance thereof and also which results in inaccurate air-fuel ratio feedback control made by an oxygen sensor provided in the exhaust system, causing poor fuel economy.
In order to eliminate these disadvantages, a split-type internal combustion engine has been proposed which includes a plurality of cylinders split into first and second groups, an intake passage provided therein with a throttle valve and divided downstream of the throttle valve into first and second branches, the first branch leading to the first group of cylinders, the second branch leading to the second group of cylinders, the second branch provided at its entrance with a stop valve adapted to close so as to prevent air from flowing into the second group of cylinders when the engine is under low load conditions, and an exhaust gas recirculation passage for re-introduction of exhaust gases into the second group of cylinders so as to reduce the vacuum appearing in the suspended cylinders thereby reducing pumping loss therein.
One difficulty with such a split-type internal combustion engine is that exhaust gases recirculated and charged in the second branch of the intake passage flow through the stop valve into the first branch thereof so as to cause unstable fuel combustion in the first group of cylinders at the moment when the engine is shifted from a split cylinder mode to a full cylinder mode. This results in poor engine performance and poor driving feel.